Strength resins are polymers generally added at the wet end of a papermaking process to the cellulosic slurry, prior to the formation of the paper mat or sheet, to improve the strength characteristics of the paper product.
U.S. Pat. No. 3,556,932, Coscia et al., which is incorporated hereinto by reference, describes ionic-hydrophilic vinylamide strength resin polymers for paper which have --CHOHCHO substituents and which thermoset or cure rapidly at common paper-making temperatures (190.degree.-250.degree. F. at the drying section, although 70.degree.-90.degree. F. is stated to be sufficient) at about a neutral pH (pH 6 to 7) to impart excellent wet strength, and which also cure to impart very satisfactory wet strength over a broad pH range of pH 4 to 8. These polymers may be self-substantive cationic polymers or anionic polymers that are deposited on fibers by the use of alum, a technique well known in the field. Paper having wet strength resulting from the use of such polymers advantageously loses a part of such wet strength when soaked in water for a moderate time period, and loses substantially all such wet strength when soaked in alkaline water. The vinylamide units provide the sites to which the --CHOHCHO groups attach by reaction with glyoxal. The vinylamide units should comprise at least 10 mole percent of the starting material polymer, up to 50 or 75 or higher mole percent. Such polymer may be a water-soluble acrylamide/diallyldimethyl ammonium chloride copolymer, or terpolymer with vinyl acetate, and molecular weights less than 25,000 are preferred because of storage stability when reacted with glyoxal. The reaction does not go to completion. If 25 mole percent of glyoxal (based on acrylamide mer units) and a 95:5 molar ratio acrylamide:diallyldimethyl ammonium chloride polymer are used, a polymer of somewhat less than 12.5 mole percent aldehyde functionality is produced. The highest glyoxal:amide ratio for preferred wet strength is stated to be 0.20:1. It is reported that dilute aqueous solutions of the reaction product are initially clear but become hazy upon standing at room or higher temperatures The highest mole percent of --CHOHCHO substituents incorporated into mer units in this patent's working examples is about 12 or 14 mole percent.
As noted in U.S. Pat. No. 4,954,538, Dauplaise et al., issued Sept. 4, 1990, which is incorporated hereinto be reference, the solutions provided by the teachings of U.S. Pat. No. 3,556,932 above have limited shelf-lives due to the crosslinking. Commercial products provided at 10% solids will gel within 8 days at room temperature.
U.S. Pat. No. 4,603,176, Bjorkquist et al., issued Jul. 29, 1986, which is incorporated hereinto by reference, is directed to temporary wet strength resin polymers also having glyoxal-capped acrylamide polymers. The resins are described as providing to paper products, such as paper towels and tissues, good initial wet strength for the intended use of such products, and also as being advantageously subject to time dependent wet tensile decay in a septic system or the like. The choice method for improving wet strength in the papermaking field has been to supplement the formation of hydrogen bonds, which are believed to provide dry strength, with the formation of chemical bonds incapable of being broken by water, during the paper product formation process. Such resin polymers are broadly described as comprised of from 5 to 95 mole percent of a polar, non-nucleophilic mer unit, from 3 to 65 mole percent of a (C.sub.1-4 alkyl or halo)acrylamide mer unit, from 1 to 30 mole percent of a glyoxal capped (C.sub.1-4 alkyl or halo)acrylamide mer unit, and from 1 to 10 mole percent of a hydrophilic cationic mer unit. A glyoxal capped acrylamide mer unit is an acrylamide unit that is substituted at the amide nitrogen with alpha-hydroxy acetaldehyde. The stated belief is that such polymer develops wet strength through the formation of both hemiacetal bonds (co-crosslinking between cellulose hydroxyl groups and resin aldehyde groups) and amidol bonds (homo-crosslinking between primary amide groups of one resin polymer and the aldehyde groups of another resin polymer), and that the rate of wet tensile decay in paper products will be proportional to the relative number of these two types of bonds. The hemiacetal bonds contribute to a rate of decay in paper products that is believed to be orders of magnitude faster than amidol bonds at a neutral pH. The relative number of amidol bonds can be reduced by reducing the number of primary amide groups in the resin polymer (and increasing the proportion of the non-nucleophilic mer units, which reduces the number of nucleophilic sites on the polymer backbone). The resin polymer is first formed with a hydrophilic, nucleophilic monomer which serves as the site of attachment for glyoxal, which mer unit is generally a primary vinylamide, preferrably (meth)acrylamide, which when partially capped with gloxal in combination satisfies two mer unit categories of the resin polymer. For instance, a polymer is formed first of a non-nucleophilic monomer, a (meth)acrylamide, and a cationic monomer, and then it is reacted with glyoxal, which attaches at the primary amide nitrogen and caps a portion of the (meth)acrylamide. Best results are described as around 35% tensile decay at 5 minutes and around 65% at 30 minutes after saturation rewetting. The glyoxal capping reaction, as illustrated in the working examples, requires a 5% buffered solution of the initial polymer to be reacted with the glyoxal for a 24 hour reaction period at 30.degree. C. The polymer resins illustrated in this U.S. Pat. No. 4,603,176 contain only from about 3 to 15 mole percent of capped acrylamide units and have molecular weights of from about 12,000 to about 63,000.
The object of the present invention is to provide a process or method of preparing a strength resin polymer that remains stable during storage and/or transport time periods, at high actives content in fluid medium. It is an object to provide such a process or method wherein the such stability is maintained despite prolonged storage and/or transport time periods, and/or despite being subjected to harsh environmental conditions during such storage and/or transport. It is an object of the present invention to provide such a process or method wherein high densities of pendant --CHOHCHO functionality may be easily achievable on the polymer without significant loss of such stability at high actives content. It is an object of the present invention to provide such a process or method wherein resin polymers of high molecular weight can be prepared without significant loss of such stability at high actives content. The object of the present invention is also to so provide a strength resin polymer that has excellent wet strength improvement properties together with the property of providing high wet strength to dry strength ratios. It is also an object of the present invention to provide such a resin polymer that provides an at least partially reversable wet strength. It is an object of the present invention to provide a strength resin polymer having a high density of bonding sites. It is an object of the present invention to provide a strength resin polymer having a high molecular weight. It is also an object to provide a paper composition containing such strength resin polymers. These and other objects of the present invention are described in more detail below.